Blast-induced traumatic brain injury (TBI) is the leading injury among military personnel who were serving in recent conflicts in Afghanistan and Iraq. Post-concussive syndrome with persistent long term cognitive impairment frequently occurs even after mild TBI, which directly impacts the health and safety of service members. Accurate diagnosis, assessment, and prognosis of structural and/or functional brain injury are extremely important and urgently needed. Unfortunately, current diagnosis and assessment for mild TBI are mainly based on self reported history, subjective measures and clinical findings. Conventional structural imaging studies such as CT or MRI have a very limited role in evaluation of mild TBI. Recently, several studies have demonstrated that Positron Emission Tomography (PET) is a reliable tool for detection of regional cerebral metabolic (RCM) changes in mild-TBI and have potential to objectively assess the functional impairment of mild TBI. PET, due to its biochemical sensitivity and quantitative measurability, has been found being able to detect subtle changes in RCM even without apparent structural changes. Despite the promising potential of PET in mild TBI evaluation, conclusion from prior studies has been limited due to variability in pattern of abnormal findings, partly due to other comorbidities such as Post Traumatic Stress Disorder (PTSD) which introduce confounding findings in clinical, neurobehavioral, and imaging presentation. In this project we will evaluate the regional cerebral metabolism changes in patients who had sustained post-concussion syndrome due to blast-induced mild TBI. PTSD patients will be included to understand the confounding findings. A total number of 210 individuals will be recruited in this study with 70 in each group (TBI; PTSD; healthy control). We will measure RCM and conduct standard neuropsychological assessments for all individuals. Appropriate statistical analyses will be applied to compare RCM and cognitive function/neurobehavioral findings among different groups. We will determine whether there is any correlation between RCM and neurobehavioral findings. The result of this study will help us understand the pathophysiological changes, especially the metabolic changes of TBI. The severity and extent of abnormal RCM findings will serve as an objective measurement for clinical evaluation, and possibly prognosis of mild TBI.